I use elk 7.10.2 with report-scheduler plugin 220.127.116.11.
When I define a report on a recent dashboard (elk 6, elk7) i.e having an id like :bfb9fc80-5df1-11ea-b16e-c34c7ff35656, the report creation works.
When I define a report on a old dashboard coming from old version elk4,5 i.e having an id like :xxx_yyy_zzz the report creation fails:
error: Bad Request
message: saved object with id dashboard:ard:xxx_yyy_zzz does not exist